Handheld, portable radios are designed to be small and compact to allow users conveniently to carry and use the portable radios. Due to size and other design considerations, the on-board antennas for portable radios often provide only marginal transmission characteristics and radiated signal strength. The transmission characteristics of a portable radio are also substantially diminished when the portable radio is used in a vehicle. To overcome the poor transmission characteristics of a portable radio operating in a vehicle, the portable radio can be coupled to a vehicle or auxiliary antenna which has improved transmission characteristics. By coupling a portable radio to a more effective auxiliary antenna, the radiated signal strength of the portable radio can be substantially improved.
In the past, portable radios have been coupled to auxiliary antennas in various different ways. For example, a portable radio can be provided with a dedicated, auxiliary-antenna connector and switch on the portable radio for providing connection to the auxiliary antenna. The auxiliary-antenna connector connects with a mating connector leading to the auxiliary antenna, and the switch functions to switch between the on-board antenna and the auxiliary-antenna connector. The problem with this type of coupling is that additional hardware and space are required in the radio for the auxiliary-antenna connector and switch.
U.S. Pat. No. 4,220,955, issued to Frye, on Sep. 2, 1980, discloses another coupling arrangement between a portable radio and auxiliary antenna. Disclosed is an antenna coupler having a helical coil that couples a helical, on-board antenna on a portable radio to an auxiliary antenna using inductive coupling. The antenna coupler of U.S. Pat. No. 4,220,955 provides a relatively complicated antenna coupler where the helical coil in the antenna coupler has a length, diameter, and number of turns based on the operating frequency of the portable radio.